Deltaontwikkeling : de betekenis van de weg- en waterbouwkunde voor de ontwikkeling van deltagebieden

Rede Landbouwuniversiteit, 12 maart 198

Túnel bajo el agua

This article describes the results obtained by the Dutch Central Institute of Scientific Research when studying the problem of underwater tunnels. This Institute has sought to establish general specifications for the design of this type of tunnels, so that they shall be more efficient than those at present in use. Firstly, a study was made of the strength, waterproof qualities, and most rational type of ventilation, with a view to drawing up of general principles on this aspect of the design. An analysis is given of the shape, suitable cross sections and constructive methods, and these are compared with those adopted in the construction of underwater tunnels at present in use in America. Particular attention has been given to designs which are largely based on prefabrication and the use of factory made structural parts, and also on the transport and erection of these at the site. The author goes on to discuss the interplay of soil and water pressures, buoyancy forces, and cases in which the tunnel is subjected to important bending and shear forces on particular sections of it. In this connection care is taken to give the theoretical considerations a ready practical interpretation. In particular, a description is given of special distance pieces which connect the two tubes making up a tunnel, also the system of joining two sections of the tube, and the measure of watertightness and flexibility which such joints provide. Finally, a summary is given of the theoretical foundations, as well as a number of observations and items of advise, which are applicable to these types of structures.El ingeniero A. Eggink describió, en un trabajo publicado últimamente, un nuevo tipo de túnel bajo el agua para canalizar la circulación rodada. En este trabajo se desarrolla un análisis sobre las consideraciones que han jugado un papel predominante en la redacción del proyecto de ejecución de un túnel subacuo. Acompañan al trabajo los resultados referentes a los distintos cálculos realizados por el Instituto Central de Investigaciones Científicas de Holanda. En este trabajo se analizan los resultados obtenidos de un estudio teórico llevado a cabo por el Instituto Central de Investigaciones Científicas de Holanda sobre túneles sumergidos en el agua, con objeto de establecer normas generales para proyectar este tipo de túneles con ventajas sobre los actualmente existentes. En primer lugar se estudian la resistencia, impermeabilización y ventilación más racionales para la redacción del proyecto de ejecución. Seguidamente se analiza la forma, secciones apropiadas y métodos constructivos, comparándolos con los utilizados en la construcción de otros túneles de esta naturaleza en América. Son objeto de particular atención los procedimientos en los que la línea general constructiva se basa en la prefabricación y montaje de elementos estructurales preparados en taller, su transporte y colocación en obra. Como detalles complementarios el autor se extiende en consideraciones del juego de presiones del suelo yagua, empujes de flotación y los casos en que aparecen flexiones y esfuerzos cortantes en los distintos tramos en que se puede subdividir la longitud total del túnel. Se esfuerza el autor en dar una carácter práctico a las aplicaciones que del estudio teórico se pueden derivar para la realización de la obra. También se describen: unos pasadores especiales que sirven para solidarizar los dos tubos de que se compone el túnel; las juntas entre dos trozos distintos del mismo, su impermeabilización y la flexibilidad que con ellos se logra. Finalmente, el autor resume su exposición teórica con una serie de observaciones y consejos aplicables a estas construcciones

Burrow ventilation in the tube-dwelling shrimp Callianassa subterranea (Decapoda: thalassinidea). III. Hydrodynamic modelling and the energetics of pleopod pumping.

The process of flow generation with metachronally beating pleopods in a tubiform burrow was studied by designing a hydrodynamic model based on a thrust-drag force balance. The drag of the tube (including the shrimp) comprises components for accelerating the water into the tube entrance, for adjusting a parabolic velocity profile, for accelerating the flow into a constriction due to the shrimp's body and another constriction due to the extended tail-fan, for shear due to separation and for the viscous resistance of all tube parts. The thrust produced by the beating pleopods comprises components for the drag-based thrust and for the added-mass-based thrust. The beating pleopods are approximated by oscillating flat plates with a different area and camber during the power stroke and the recovery stroke and with a phase shift between adjacent pleopod pairs. The added mass is shed during the second half of the power stroke and is minimized during the recovery stroke. A force balance between the pleopod thrust and the tube drag is effected by calculating the mean thrust during one beat cycle at a certain flow velocity in the tube and comparing it with the drag of the tube at that flow velocity. The energetics of the tube and the pump are derived from the forces, and the mechanical efficiency of the system is the ratio of these two. Adjusted to standard Callianassa subterranea values, the model predicts a mean flow velocity in the tube of 1.8 mm s-1. The mean thrust force, equalling the drag, is 36. 8 microN, the work done by the pleopod pump per beat cycle is 0.91 microJ and the energy dissipated by the tube system is 0.066 microJ per cycle. The mechanical efficiency is therefore 7.3 %. Pump characteristics that may be varied by the shrimp are the beat frequency, the phase shift, the amplitude and the difference in pleopod area between the power and recovery strokes. These parameters are varied in the model to evaluate their effects. Furthermore, the moment of added mass shedding, the distance between adjacent pleopods, the number of pleopods and the total tube drag were also varied to evaluate their effects